Three-dimensional integrated circuits with protection layers

ABSTRACT

A semiconductor structure includes a first die comprising a first substrate and a first bonding pad over the first substrate, a second die having a first surface and a second surface opposite the first surface, wherein the second die is stacked on the first die and a protection layer having a vertical portion on a sidewall of the second die, and a horizontal portion extending over the first die.

This application is a divisional of U.S. patent application Ser. No.11/641,324, filed Dec. 19, 2006, and entitled “Three-DimensionalIntegrated Circuits with Protection Layers,” which application is herebyincorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to integrated circuits, and moreparticularly to structures and formation methods of three-dimensionalintegrated circuits.

BACKGROUND

Since the invention of the integrated circuit, the semiconductorindustry has experienced continued rapid growth due to continuousimprovements in the integration density of various electronic components(i.e., transistors, diodes, resistors, capacitors, etc.). For the mostpart, this improvement in integration density has come from repeatedreductions in minimum feature size, which allows more components to beintegrated into a given area.

These integration improvements are essentially two-dimensional (2D) innature, in that the volume occupied by the integrated components isessentially on the surface of the semiconductor wafer. Although dramaticimprovement in lithography has resulted in considerable improvement in2D integrated circuit formation, there are physical limits to thedensity that can be achieved in two dimensions. One of these limits isthe minimum size needed to make these components. Also, when moredevices are put into one chip, more complex designs are required.

An additional limit comes from the significant increase in the numberand length of interconnections between devices as the number of devicesincreases. When the number and length of interconnections increase, bothcircuit RC delay and power consumption increase.

Three-dimensional (3D) integrated circuits (ICs) are therefore createdto resolve the above-discussed limitations. In a typical formationprocess of 3D IC, two wafers, each including an integrated circuit, areformed. The wafers are then bonded with the devices aligned. Deep viasare then formed to interconnect devices on the first and secondsubstrates.

Much higher device density has been achieved using 3D IC technology, andup to six layers of wafers have been bonded. As a result, the total wirelength is significantly reduced. The number of vias is also reduced.Accordingly, 3D IC technology has the potential of being the mainstreamtechnology of the next generation.

Conventional methods for forming 3D IC also include die-to-waferbonding, wherein one or more die is bonded to a wafer. An advantageousfeature of the die-to-wafer bonding is that the size of dies may besmaller than the size of chips on the wafer. During a typicaldie-to-wafer bonding process, spaces will be left between the dies. Thespaces are typically filled with a coating, such as spin-on-glass. Thewafer and the dies on wafer are then sawed. However, the conventionaldie-to-wafer bonding processes have drawbacks. The coating processintroduces moisture and chemical contamination, which degrades the bondsbetween the dies and the wafer. A new die-to-wafer bonding process istherefore needed to solve this problem.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a semiconductorstructure includes a first die comprising a first substrate and a firstbonding pad over the first substrate; a second die having a firstsurface and a second surface opposite the first surface, wherein thesecond die is stacked on the first die and a protection layer having avertical portion on a sidewall of the second die; and a horizontalportion extending over the first die.

In accordance with another aspect of the present invention, asemiconductor structure includes a wafer comprising a first die and asecond die, wherein each of the first and second dies includes a firstsubstrate and a first bonding pad on a top surface of the firstsubstrate. The semiconductor structure further includes a third die anda fourth die each having a first surface and a second surface oppositethe first surface, wherein the third die is stacked on the first die,and the fourth die is stacked on the second die. Each of the third andthe fourth dies includes a second substrate; an active device in thesecond substrate; an interconnect structure over the second substrate; asecond bonding pad on the first surface and connected to theinterconnect structure, wherein the second bonding pad of the third dieis bonded to the first bonding pad of the first die, and wherein thesecond bonding pad of the fourth die is bonded to the first bonding padof the second die; and a contact pad on the second surface andelectrically connected to the second bonding pad. The semiconductorfurther includes a protection layer in a space between the third and thefourth dies, wherein the protection layer has vertical portions onsidewalls of the third die and the fourth die and a horizontal portionextending from over the first die to over the second die, and a coatingon the protection layer, wherein the coating fills a remaining spacebetween the third and the fourth dies.

In accordance with yet another aspect of the present invention, asemiconductor structure includes a supporter free from active devices,wherein the supporter comprises a first substrate and a first bondingpad over the first substrate. The semiconductor structure furtherincludes a die having a first surface and a second surface opposite thefirst surface, wherein the die is stacked on the supporter, and aprotection layer having a vertical portion on a sidewall of the seconddie and a horizontal portion extending over the supporter. The dieincludes a second substrate; an active device in the second substrate;an interconnect structure over the second substrate; a second bondingpad on the first surface and connected to the interconnect structure,wherein the second bonding pad is bonded to the first bonding pad of thesupporter; and a contact pad on the second surface and electricallyconnected to the second bonding pad.

In accordance with yet another aspect of the present invention, a methodfor forming a semiconductor structure includes providing a wafercomprising a first die and a second die, each comprising a firstsubstrate; and a first bonding pad on a top surface of the wafer,forming a third die and a fourth die, each having a first surface and asecond surface opposite the first surface wherein the third die isstacked on the first die and the fourth die is stacked on the seconddie; forming a protection layer in a space between the third and thefourth dies, wherein the protection layer has vertical portions onsidewalls of the first and the second dies and a horizontal portionextending from over the first die to over the second die; and forming acoating on the protection layer; and filling a remaining space of thespace between the third and the fourth dies.

In accordance with yet another aspect of the present invention, a methodfor forming a semiconductor structure includes providing a wafercomprising a first die and a second die, each comprising a firstsubstrate; a first bonding pad on a top surface of the first substrate;and forming a third die and a fourth die each having a first surface anda second surface opposite the first surface. The steps of forming eachof the third and the fourth dies comprise providing a second substrate;forming an active device in the second substrate; forming a deepdielectric plug in the second substrate; forming an interconnectstructure physically connected to the deep dielectric plug; and forminga second bonding pad on the first surface, wherein the second bondingpad is connected to the interconnect structure. The method furtherincludes stacking the first die to the third die with the first bondingpad of the first die bonded to the second bonding pad of the third die;stacking the second die to the fourth die with the first bonding pad ofthe second die bonded to the second bonding pad of the fourth die;forming a protection layer on the third and the fourth dies and in aspace between the third and the fourth dies, wherein the protectionlayer has vertical portions on sidewalls of the third and the fourthdies and a horizontal portion extending from over the first die to overthe second die; forming a coating on the protection layer; filling aremaining space of the space between the third and the fourth dies;polishing the coating, the protection layer, and the first substrate ofthe third and the fourth dies to expose the deep dielectric plug;replacing the deep dielectric plugs of the third and the fourth dieswith a conductive material to form through-silicon vias; and formingcontact pads electrically interconnected to the through-silicon vias ofthe third and fourth dies.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1 through 11 are cross-sectional views of intermediate stages inthe manufacturing of an embodiment of the present invention, whereindies are bonded on a wafer;

FIG. 12 illustrates an additional layer of dies stacked on adie-to-wafer bonding structure;

FIG. 13 illustrates an embodiment of stacked dies, wherein the bondingbetween bonding pads are solder bumps;

FIG. 14 illustrates the stacking of dies on a supporter wafer; and

FIGS. 15A through 15E illustrate different stacking schemes, whereindies having different sizes are stacked.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The intermediate stages of a novel method for forming athree-dimensional integrated circuit (3DIC) are illustrated in FIGS. 1through 11. The variations of the preferred embodiments are thendiscussed. Throughout the various views and illustrative embodiments ofthe present invention, like reference numbers are used to designate likeelements.

FIGS. 1 through 5 illustrate the preparation of a die, which will bestacked on a wafer. Referring to FIG. 1, a semiconductor substrate 10 isprovided. Shallow trench isolation (STI) regions 12 and openings 16 arethen formed in semiconductor substrate 10, as illustrated in FIG. 2A. Asis known in the art, STI regions 12 may be formed by forming openings insemiconductor substrate 10 and filling the openings with a dielectricmaterial such as high-density plasma (HDP) oxide. Preferably, theformation of openings 16 includes forming and patterning mask layer 14,which may be a photo resist, on substrate 10, and etching semiconductorsubstrate 10. The etching may be a dry etching with the assist ofplasma. Alternatively, openings 16 may be formed by laser drilling.Openings 16 preferably have a depth greater than about 5 μm, and morepreferably between about 50 μm and about 50 μm. Mask layer 14 is thenremoved.

Referring to FIG. 3A, openings 16 are filled with a material havingdifferent etching characteristics from substrate 10, preferably adielectric material, resulting in deep plugs 18. For simplicity, deepplugs 18 are alternatively referred to as deep dielectric plugs 18. FIG.4 illustrates the formation of integrated circuits, which includesactive devices, at a surface portion of semiconductor substrate 10,wherein a metal-oxide-semiconductor (MOS) device is used to symbolizeintegrated circuit 20. Alternatively, integrated circuit 20 may beformed before the formation of deep dielectric plugs 18, as isillustrated in FIGS. 2B and 3B.

Referring to FIG. 5, an inter-layer dielectric (ILD) 22 is formed on thestructure shown in FIG. 4, and contact plugs 24 are formed in ILD 22.Some of the contact plugs 24 physically contact deep dielectric plugs18, while the others (not shown) may be electrically connected todevices in integrated circuit 20. Contact plugs 24 may be formed ofmetallic materials including tungsten, copper, aluminum, silver, andother commonly used materials. An interconnect structure 30 is thenformed, which includes metallization layers formed in dielectric layers.Conductive features 28 are schematically illustrated to represent metallines in metallization layers and the connecting vias, and a dielectriclayer 26 is used to symbolize a plurality of inter-metal dielectriclayers. The inter-metal dielectric layers may include low-k materialswith k values less than about 3.5, such as carbon-doped silicon oxide.Interconnect structure 30 interconnects deep dielectric plugs 18 tobonding pads 32, which are located on the top of interconnect structure30. Bonding pads 32 may include metals such as copper, tungsten andaluminum, alloys such as CuSn, AuSn, InAu, PbSn, and combinationsthereof. Horizontal dimension of each of the bonding pads 32 ispreferably less than about 50*50 μm². The formation processes for ILD22, contact plugs 24, interconnect structure 30, and bonding pads 32 arewell known in the art, thus are not repeated herein. The resultingstructure is referred to as a die 36. One skilled in the art willrealize that die 36 is preferably fabricated as a part of a wafer andsawed from the wafer after the interconnect structures and bonding padsare formed.

Referring to FIG. 6, die 36 is flipped upside-down and placed against awafer 40. For simplicity, only two dies 42 and 44 in wafer 40 areillustrated, although a wafer will contain a plurality of identicaldies. In FIG. 6, die 36 is placed against die 42, and another die 38,which has an identical structure as die 36, is placed against die 44.Preferably, each of the plurality of dies in wafer 40 is bonded to atleast one die. Such a bonding is typically referred to as a die-to-waferbonding. Similar to die 36, dies 42 and 44 may also include integratedcircuits 46 formed therein, and bonding pads 48 on top of the dies.Bonding pads 48 on dies 42 and 44 are aligned with bonding pads 32 ondies 36 and 38, respectively, and may be bonded using direct copperbonding (also referred to copper-to-copper bonding, although bondingpads 32 and 48 may include other elements than copper), fusion bonding,diffusion bonding or solder bump bonding. In an exemplary direct copperbonding process, dies 36 and 38 are bonded to wafer 40 by applying ahigh pressure, which may be about 10 pounds per square inch (psi) toabout 100 psi. The temperature is preferably between about 300° C. andabout 500° C.

There are typically gaps 50 between neighboring bonding pads 32 and 48.These gaps 50 may be filled with gap-filling materials such asunderfills, which are commonly used in packaging art. Alternatively,gaps 50 may be left as they are, so that air insulations are formedbetween bonding pads.

Referring to FIG. 7, a protection layer 52 is blanket formed, preferablyconformably, on sidewalls of dies 36 and 38. It is noted that at corners54, protection layer 50 is preferably continuous so that pads 32, 48 andthe respective bonds are fully sealed from external environment.Protection layer 52 is preferably formed of dense dielectric layers thathave good insulating ability, so that it will prevent moisture andchemicals in external environment from penetrating it and reaching bondsbetween pads 32 and 48. The preferred materials of protection layer 52include diamond film, amorphous carbon, silicon oxide, silicon nitride,silicon carbide, silicon oxynitride, silicon oxycarbide, andcombinations thereof. The thickness T of protection layer 52 ispreferably between about 50 Å and about 2000 Å, although the optimumthickness depends on its density and conformity on sidewalls of dies 36and 38. The preferred formation methods include physical vapordeposition, dipping, chemical vapor deposition (CVD) methods such asplasma enhanced CVD (PECVD), low pressure CVD (LPCVD), atomic layer CVD(ALCVD), and the like.

FIG. 8 illustrates the filling of a coating 56 into the remainingspacing between dies 36 and 38. The filling of a coating 56 into theremaining spacing between dies 36 and 38 can be performed by spincoating, stencil or dispensing. The preferred gap-filling materialsinclude spin-on-glass, polyimide, bis-benzocyclobutene (BCB), and othercommonly used gap-filling materials/polymers. The formation methodsinclude spin coating, dipping, and the like. Preferably, coating 56 canendure a relatively high temperature, for example, about 400° C., whichmay occur in subsequent processes. The material density of coating 56 ispreferably smaller than that of protection layer 52.

During the subsequent processes, coating 56 may release contaminationssuch as moisture and/or chemicals. These materials will corrode bondingpads 32 and 48 and degrade their bonds. In addition, coating 56 istypically not dense enough to prevent moisture and chemicals frompenetrating through it from outside. Protection layer 52 thus protectsmoisture and/or chemicals from reaching bonding pads 32, 48 and theirbonds. The reliability of the stacked dies is thus improved.

Referring to FIG. 9, a chemical mechanical polish (CMP) is preformed toremove portions of coating 56, protection layer 52 and substrate 10,until deep dielectric plugs 18 are exposed. Alternatively, grinding,plasma etching, or wet etching may be used for the exposure of deepdielectric plugs 18.

FIG. 10 illustrates the formation and patterning of a passivation layer58. Passivation layer 58 preferably includes dielectric materials suchas silicon oxide or silicon nitride formed using PECVD, although othermaterials can also be used. Openings 60 are then formed to expose deepdielectric plugs 18. Deep dielectric plugs 18 are then removed so thatopenings 60 extend downward to expose contact plugs 24. Preferably, thepatterning of passivation layer 58 and the removal of deep dielectricplugs 18 comprise plasma etching. However, wet etching can also beperformed.

Referring to FIG. 11, openings 60 are filled with conductive materials.Preferably, diffusion barrier layers (not shown), which may includemetal nitrides such as tantalum nitride (TaN) and titanium nitride(TiN), and metals such as Ta, Ti, CoW, are formed in openings 60. Aconductive material, which may include copper, copper alloy, tungsten,aluminum, aluminum alloy, silver, and combinations thereof, is thenfilled into the remaining portions of openings 60, formingthrough-silicon vias 64 in substrate 10 and contact pads 66 inpassivation layer 58. The conductive material can be formed usingelectrical plating or a CVD method.

The wafer shown in FIG. 11 may then by sawed along a line A-A′ toseparate dies. The resulting dies are stacked dies. It is appreciatedthat if more than two layers of dies need to be stacked, additional diesmay be processed using similar steps as shown in FIGS. 1 through 5, andthen stacked on the wafer shown in FIG. 11. An exemplary embodiment isshown in FIG. 12, wherein dies 68 and 70 are further stacked. Oneskilled in the art will realize the respective formation steps.Sidewalls of dies 68 and 70 are preferably protected by an additionalprotection layer 69, which comprises essentially the same materials asprotection layer 52. Also, a coating 71, which comprises essentially thesame materials as coating 56, is formed.

In the embodiments discussed in the preceding paragraphs, dies 36 and 38are bonded to wafer 40 using copper-to-copper bonding. In alternativeembodiments, solder bumps may be used. In FIG. 13, solder bumps 72 areused to electrically connect bonding pads 32 of die 36 and bonding pads48 of die 42. Similar to the case illustrated in FIG. 6, gaps 50 mayeither be air insulations, or filled with gap-filling materials. Inalternative embodiments, fusion bonding or diffusion bonding may be usedto form stacked dies, wherein a protection layer as shown in FIG. 11 isdesired regardless of the bonding methods.

In a further embodiment of the present invention, as illustrated in FIG.14, wafer 40 (refer to FIG. 6) may be replaced with a supporter wafer74, which does not contain active devices therein. In this case, bondingpads 32 of die 36 are connected to bonding pads 76 on supporter wafer74. It is to be noted that some of bonding pads 76 can also beelectrically interconnected through metal lines, which are not in thesame plane as the cross-sectional view, and thus are not shown. Thebonding pads 76 are eventually connected back to interconnect structureor devices in die 36. Therefore, supporter wafer 74 not only provides astructural support, but also provides electrical routing ability. Thesubstrate of the support wafer 74 may comprise a semiconductor materialsuch as silicon, or a dielectric material such as glass. The supportwafer 74 also provides the function of heat dissipation.

An advantageous feature of die-to-wafer bonding is that dies do not haveto have the same size as dies on wafers. This provides significantflexibility. FIGS. 15A through 15E illustrate some exemplary schemes forutilizing this advantageous feature. In FIG. 15A, more than one die maybe stacked side-by-side on other dies. In FIG. 15B, in each layer, thestacked dies may have increasingly smaller sizes. In FIGS. 15C and 15D,one or more die may be formed between two dies with greater sizes. FIG.15E illustrates a third level die having a greater size than a secondlevel die. One skilled in the art will realize that there are moreavailable schemes. With more levels of dies stacked, the reliability ofbonds between dies becomes a more significant factor to the overallreliability of stacked dies. The formation of protection layer servesthe requirement for higher demands in reliability.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method comprising: providing a wafer comprising a first die and asecond die, each comprising a first bonding pad on a top surface of thecorresponding first and second dies; providing a third die and a fourthdie each comprising a first surface and a second surface opposite thefirst surface; stacking the third die on the first die, and the fourthdie on the second die, wherein a second bonding pad on the first surfaceof each of the third and the fourth dies is bonded to the first bondingpad of the first die through flip-chip bonding, and wherein the thirddie and the fourth die have a gap therebetween; forming a protectionlayer comprising vertical portions on sidewalls of the third and thefourth dies, and a horizontal portion directly over the first die andthe second die; and forming a coating on the protection layer andfilling a remaining space of the space between the third and the fourthdies.
 2. The method of claim 1, wherein the steps of providing the thirdand the fourth dies and stacking the third and the fourth dies comprise:for each of the third and fourth dies, providing a substrate comprisingan active device thereon; forming an interconnect structure over thesubstrate; forming the second bonding pad on the first surface andelectrically coupled to the interconnect structure; and formingthrough-silicon vias in the substrate.
 3. The method of claim 2, whereinthe steps of providing the third and the fourth dies and stacking thethird and the fourth dies further comprise: forming deep plugs extendingfrom top surfaces of substrate into the substrate; after the protectionlayer and the coating are formed, polishing the substrate to expose thedeep plugs; removing the deep plugs to form openings; and performing thestep of forming the through-silicon vias by filling the openings with aconductive material.
 4. The method of claim 1 further comprising asawing process to separate the first die and the third die from thesecond die and the fourth die, wherein the horizontal portion of theprotection layer and the coating are sawed through, and wherein thevertical portions of the protection layer are not sawed.
 5. The methodof claim 1, wherein the step of forming the protection layer isperformed using a method selected from the group consisting essentiallyof physical vapor deposition, dipping, plasma enhanced chemical vapordeposition (CVD), low pressure CVD, and atomic layer CVD.
 6. The methodof claim 1, wherein the step of forming the coating is performed using amethod selected from the group consisting essentially of spin coating,stencil, and dipping.
 7. The method of claim 1, wherein the horizontalportion and the vertical portions of the protection layer form acontinuous layer with no break therebetween.
 8. The method of claim 1,wherein the protection layer has a density greater than a density of thecoating.
 9. The method of claim 1 further comprising forming activedevices at surfaces of the substrates of the first and the second dies,and forming interconnect structures over the substrates of the first andthe second dies.
 10. The method of claim 1, wherein the first and thethird dies are stacked by forming a bond between the first bonding padof the first die and the second bonding pad of the third die, whereinthe bond is selected from the group consisting essentially of fusionbonding, diffusion bonding, and solder bonding.
 11. A method comprising:providing a wafer comprising a first die and a second die; stacking athird die to the first die through flip-chip bonding, wherein a firstgap is formed between the first die and the third die; stacking a fourthdie to the second die through flip-chip bonding, wherein a second gap isformed between the second die and the fourth die; forming a protectionlayer on top surfaces of, and extending on sidewalls of, the third andthe fourth dies, wherein the protection layer seals the first and thesecond gaps from aside; filling a coating on the protection layer andfilling an entirety of the space between the third and the fourth dies;and performing a polish to remove portions of the protection layerdirectly over the top surfaces of the third and the fourth dies, until atop portion of each of the third and the fourth dies are removed. 12.The method of claim 11, wherein the third and fourth dies comprisedielectric vias in semiconductor substrates, and wherein after the stepof performing the polish, the dielectric vias are exposed.
 13. Themethod of claim 12 further comprising replacing the dielectric vias withthrough-silicon vias.
 14. The method of claim 12, wherein the dielectricvias are embedded in the semiconductor substrates, with top layers ofthe semiconductor substrates covering the dielectric vias, and whereinin the step of performing the polish, the top layers of thesemiconductor substrates are removed.
 15. The method of claim 11,wherein during the step of stacking the third die to the first die, abonding pad at a surface of the first die is bonded to a bonding pad ata surface of the third die through metal-to-metal bonding.
 16. Themethod of claim 11, wherein the protection layer has a density greaterthan a density of the coating.
 17. A method comprising: providing awafer comprising a first die and a second die, each comprising: a firstsubstrate; and a first bonding over the first substrate; forming a thirddie and a fourth die comprising: providing a second substrate; forming adeep dielectric plug in the second substrate; forming an interconnectstructure physically connected to the deep dielectric plug; and forminga second bonding pad over, and electrically coupled to, the interconnectstructure; stacking the first die to the third die with the firstbonding pad of the first die bonded to the second bonding pad of thethird die; stacking the second die to the fourth die with the firstbonding pad of the second die bonded to the second bonding pad of thefourth die; forming a protection layer, wherein the protection layercomprises vertical portions on sidewalls of the third and the fourthdies, and horizontal portions directly over the first and the seconddies; forming a coating on the protection layer and filling a remainingportion of the space horizontally between the third and the fourth dies;polishing the coating, the protection layer, and the second substratesof the third and the fourth dies to expose the deep dielectric plugs;replacing the deep dielectric plugs of the third and the fourth dieswith a conductive material to form through-silicon vias; and formingcontact pads electrically coupled to the through-silicon vias.
 18. Themethod of claim 17 further comprising a sawing process to separate thefirst die and the third die from the second die and the fourth die. 19.The method of claim 18 further comprising, before the step of sawing,stacking a fifth die on the third die, and forming an additionalprotection layer on a sidewall of the fifth die.